This invention relates generally to gas insulated transmission lines, and more particularly to support means for insulatably supporting the inner conductor within the outer sheath of the transmission line, which support means utilizes a barrier to prevent the migration of arc products and contamination past the support means.
Compressed gas insulated transmission lines are being used in an ever-increasing scale in recent years due to the desirability of increasing safety, their environmental acceptability, problems in acquiring right-of-way for overhead lines, higher power loads required by growing metropolitan areas and growing demands for electrical energy. Compressed gas insulated transmission lines installed to date have typically comprised a hollow outer sheath, an inner conductor within the sheath, a plurality of insulating spacers which support the conductor, and a compressed gas such as sulfur hexafluoride or the like in the sheath to insulate the inner conductor from the outer sheath. The typical assembly has been fabricated from relatively short sections of hollow cylindrical ducts or tubes into which the conductor and insulators are inserted. The assembly is usually completed in the factory, and the sections are welded or otherwise secured together in the field to form transmission lines. It is also known to provide a particle trap in compressed gas insulated transmission lines as is disclosed in the patent to Trump, U.S. Pat. No. 3,515,939. The particle trap of Trump is utilized to allow conducting or semiconducting particles which could adversely affect the breakdown voltage of the dielectric gas to move from locations where such particles would cause breakdowns to areas where the particles are deactivated.
One problem which has arisen in the use of such compressed gas insulated lines is that, occasionally, not all of the particles are captured in the particle traps, which sometimes may collect on the insulating spacer surfaces and initiate high voltage flashover. To decrease the probability of this happening, it is desirable that the spacer surface area upon which the particles could collect be made as small as possible. However, the use of such minimum-surface area spacers itself presents a problem; if a failure occurs in the line, arc products and other contamination particles may be generated, and these contamination particles and arc products can migrate from the section of the transmission line where the failure occurred to adjacent sections. This movement of arc products and contamination throughout the transmission line may then cause subsequent substantial damage. One manner of avoiding this problem is to use conical or disc spacers which substantially fill the cross-sectional area of the transmission line, and which block progress of contamination or arc products along the line. But the use of such spacers presents again the problem of having a large surface area upon which particles may collect and initiate flashover.
The above-described problems have been minimized by the use of two types of spacers within the gas insulated transmission line; a first spacer having minimum surface area, and a second spacer which substantially fills the cross-sectional area between the inner and outer electrical conductors. This is the concept taught and described in the patent to Cookson et al, U.S. Pat. No. 4,105,859. A new type of gas insulated transmission line is presently being investigated which is called flexible or semi-flexible gas insulated transmission lines. These types of transmission lines utilize a corrugated outer sheath and a flexible, to a degree, inner conductor to provide a degree of flexibility to the transmission line to enable it, for example, to change directions without the use of accessory equipment such as elbows. In these new types of transmission lines, the insulating spacers which are utilized to support the inner conductor, or conductors, within the outer sheath are typically of a low dielectric constant material, and are more closely spaced together than corresponding insulators in the rigid-type systems. For example, whereas in the rigid type system an insulator may be disposed every 20 feet, the flexible or semi-flexible transmission lines utilize insulators spaced on the order of 5 or 6 feet apart. Thus, as with the rigid system, the use of minimum-surface area insulators is encouraged, while the use of barrier-type insulators to prevent arc product migration is desirable. However, the conical or disc barrier-type insulators utilized in the rigid systems typically have an inferior flashover voltage characteristic when compared to the insulator utilized in the remainder of the transmission line. Thus, what is desirable is a novel type of barrier insulator which may be utilized in flexible or semi-flexible gas insulated transmission lines.